Patient security systems have been used in hospital maternity wards as a deterrent to criminal infant abductions. Other uses of such identification and locations systems include monitoring patients in pediatric and neurologic centers, as well as in nursing homes to reduce the likelihood of an Alzheimer's or head trauma patient wandering out of the facility.
Generally, patients wear a small electronic tag. The tag may be located within a bracelet placed around the patients wrist or ankle, and includes a transponder (transmitter and receiver). The receiver monitors a selected frequency, and is activated when a signal on that frequency is detected. The activating signal originates from an antenna typically located near a door or other exit. The activation occurs when the tag approaches the antenna. Upon activation, the tag transmits a signal to a receiver and control unit, indicating that the wearer of the tag might be leaving the area. The door may then be automatically locked, and/or the staff may be alerted by an alarm such as a bell or flashing lights. The signal transmitted by the tag can include information such as patient name, location of the patient etc. Also, some prior art systems will sound the alarm if the bracelet is cut, to indicate that the patient no longer is wearing the tag.
However, the prior art systems have drawbacks. One such drawback is that the tag may slip off of the patients leg or arm, either intentionally or inadvertently. This is particularly true for infants where weight loss shortly after birth is common. Thus, an initially snug fitting bracelet may become loose fitting. Also, prior art systems typically use a single loop antenna, which produces regions where the transmitted signal has a relatively low intensity (dead zones) in planes perpendicular to the axis of the single loop. Thus, it is possible to have a tag oriented in one or more planes and pass through a protected door without being activated. Also, the prior art systems are not useful other than as locators/trackers--they did nothing to alert personnel to potential medical problems, such as respiratory failure. Other problems with prior security tag systems include the use of high frequency signals which will activate a tag from a greater distance (thus perhaps inadvertently activating a tag) than will lower frequency signals, the need to replace batteries due to power consumption because the tag is "on" at all times, the inability to automatically check if tags are working properly, and false alarms due to random noise.
Accordingly, a security tag that may not be removed without setting off an alarm is desired. Also, such a system should employ antennas that avoid creating dead zones, which comprise security. Such a system should, preferably, be capable of providing an alarm in the event the patient suffers respiratory failure, and should be able to operate at (relatively) low frequencies, with a low standby power consumption, the ability to determine if tags are working automatically, and the ability to avoid false alarms due to random noise.